Is it permissible to marry a girl whose brother was suckled by her mother while she was a child, knowing that he was 15 years old at the time?

Allah has forbidden marriage to a sister by suckling, as He says: "And your mothers who suckled you, and your sisters from suckling," and the Prophet, peace and blessings be upon him, said: "What is forbidden by lineage is forbidden by suckling."

For suckling to be a prohibitive factor, it must occur within the first two years, and consist of five full sucklings.

So, if the girl was suckled by her paternal aunt five full sucklings within the first two years, she becomes a sister to her children by suckling, and it is forbidden for them to marry her. Otherwise, it is not forbidden. The young man should confirm with his mother the number of sucklings and the time they occurred.